If an AC voltage is provided to a load which is arranged remote of a voltage source, the output AC voltage of the voltage source does not reach the load in full. Instead, a considerable voltage drop over the electrical supply line, which connects the load to the voltage source is observed. The relevant contributions to the voltage drop over the electrical supply line are provided by the ohmic resistance and the inductive reactance of the supply line. In addition, there is also a capacitive reactance which, however, may be neglected in most applications. The ohmic resistance and the inductive reactance of the supply line are not constant so that they can not be compensated for by a constant compensation AC voltage as a necessary addition to the desired constant AC voltage at the voltage source. Instead, they vary with the total value of the alternating current conducted to the load as the total value of the voltage drop caused by the ohmic resistance is |I|* R, and the total value of the voltage drop caused by the inductive reactance is |I|*ωL.
Thus, it is known in a method of the type described at the beginning to measure the total value of the alternating current conducted to the load and to multiply this total value by an adjustable constant for selecting the compensation AC voltage. The adjustable constant is selected depending on the actual supply line. This selection is, for example, effected in that the constant is increased starting at zero until the desired constant AC voltage is provided to the load. This known method does, however, not take into consideration that the voltage drop over the supply line caused by its ohmic resistance and the voltage drop over the supply line caused by its inductive reactance are to be summed up in a vector addition to determine the total voltage drop over the supply line, and that this total voltage drop over the supply line is also a vector so that an ideal compensation voltage is achieved by means of a closed vector triangle which is formed by the drop in voltage over the supply line, the output voltage of the voltage source and the AC voltage at the load. In other words, a determination of the compensation AC voltage only depending on the total value of the alternating current conducted to the load is insufficient, if a phase angle phi between the alternating current conducted to the load and the output AC voltage of the voltage source varies, because this variation changes the direction of the vectors mentioned above and the ratio of the real component and the imaginary component of the complex AC voltage quantities.
In a further method of the type described at the beginning, in addition to the variation of the value of the compensation AC voltage depending on the total value of the alternating current conducted to the load, a compensation capacity is connected in series with the supply line connecting the load to the voltage source to compensate for the inductive reactance by means of a capacitive reactance to such an extent that the voltage drop over the electrical supply line is now only determined by its ohmic resistance which is only depending on the total value of the alternating current conducted to the load. The phase angle between the output AC voltage of the voltage source and the alternating current conducted to the load, however, depends on the inductivity of the whole system, which may vary to a considerable extend. Thus, it is impossible, to adjust the phase angle to zero by means of a constant capacitance. On the other hand, considerable dangers are incurred by the additional high capacitance in the supply line to the load.
As a further method of providing a constant AC voltage to a variable load which is arranged remote of a voltage source, a voltage drop over an electrical supply line which connects the load to the voltage source is compensated for by means of a compensation AC voltage which, if added to the constant AC voltage, results in the output AC voltage of the voltage source, the AC voltage reaching the load being measured and being used as an actual value for controlling the voltage source. This method results in a constant AC voltage at the load independently of any changes in the whole system. However, problems can occur, if measuring supply lines which also run between the voltage source and the load, are affected by disturbances. The function of the known method is lost, if any of the measuring supply lines breaks.
Thus it would be desirable to provide a method of providing a constant AC voltage to a variable load by which the compensation AC voltage is selected in such a way that it results in a constant AC voltage at the load over a greater range of variations of the load. At the same time, the method should be easily applied and implemented.